Dispensing systems for the delivery of useful agents are well known to the prior art. These systems generally deliver their agent by diffusion, for example, from an enclosed capsule or by diffusion from a multi-structured device having a wall formed of a known polymer permeable to the agent into a selected environment. However, there is a large category of agents that cannot be delivered by the prior art delivery systems because of at least one feature inherent in these devices which adversely affects their rate of release from the system or substantially prevents the release of the agent from the system. For example, many agents cannot be delivered from a diffusion controlled delivery system because their permeation rate through the rate controlling material comprising the system is too small to to produce a useful effect or in many instances the agent molecules are too big and will not diffuse through the rate controlling material forming the device. Also, there is an additional class of useful agents that cannot be satisfactorily delivered by diffusion devices because of a particular chemical characteristic of the agent. This additional class includes salts that because of their ionic character will not diffuse through most polymers and polymeric like materials and unstable polar compounds that cannot be formulated into a satisfactory composition suitable for storage and delivery from a prior art device. Prior art diffusion systems encompassed within the above discussion are typically represented by U.S. Pat. No. 3,279,996, and the like.
The prior art attempted to overcome the above mentioned adverse features by proposing devices seemingly capable of releasing a solution containing a product at a relatively constant rate. One such device is disclosed in AUSTRAL. J. EXP. BIOL., Vol. 33, pages 415 to 420, 1955. This device consists of three compartments confined in a specially constructed housing and a clamp to hold a semi-permeable membrane. The driving force of the device depends on the continual presence of a solution of an osmotically effective red dye solute that exhibits an osmotic pressure gradient against water. The red dye is contained in a partially collapsed rubber compartment and it is separated from a second compartment containing water by a semi-permeable membrane. The partially collapsed bag is housed in a glass ampoule, along with a product compartment defined by the space between the bag and one end of the glass ampoule. The distant end of the ampoule defines a water compartment. The ampoule also is provided with a drug release nipple, and in operation when the product compartment is charged with a solution of a product, water in the water compartment moves through the semi-permeable membrane into the dye solution increasing its volume in the compartment causing it to expand against the rubber providing the mechanical force necessary to eject the product solution through the nipple. It is immediately evident that this device has certain adverse features that tend to diminish its practical use. For example, the device is difficult to construct into compartments that are essentially free of leaks plus the fabrication demands of a movable material that necessitates a rigid outer housing. Another inherent disadvantage which prevented its wide acceptance by the medical community is the requirement that the product be in solution which exhibit a deleterious tendency to be released from the device by simple leaching, the use of a solution of the product which use does not permit high concentrations of the product to be embodied within the device, the demand for an osmotically effective solute other than the product, and that many products on prolonged storage in solution undergo chemical deterioration. The device is further of limited value because it must carry its own water which increases the size of the device and thusly limits its use to a few environments. Another prior art attempt to provide a product dispensing device is disclosed in U.S. Pat. No. 3,604,417. The device disclosed in this patent is similar to the earlier prior art devices, and its design requires a semi-permeable membrane, a separate osmotically effective solute, a solution of the product and additionally the presence of a movable piston. The movable piston severely restricts the shape of the device, and this device, as with the above mentioned device, has not enjoyed wide acceptance because of construction problems and the inherent features that limit the use of the device.
In prior art U.S. Pat. No. 3,146,169 there is disclosed a pharmaceutical tablet formulation comprised of a layer impermeable to both water and medication which surrounds the medication except for a hole. In this device medication is released by uncontrolled dissolution predicated upon many variables. For example, the rate of medication dissolution is pH dependent that varies according to the environment, and when it changes the solubility of the medication its rate of release is correspondingly changed. The rate of release is also stirring rate dependent and any external fluid must be in a constant of flux to cause a dissolution of the product for its release from the tablet. Hence, if there is little or no external fluid, the product will not be released from the device. Further, in this tablet the exposed surface area of the product continually changes at an uncontrolled rate to release product in changing amounts, which features seriously limit the usefulness of the tablet. Also, as the exposed surface area of medication and the length of the diffusional path change, the amount of medication released becomes unpredictable and, as such, this device cannot be relied upon for use in the management of health and disease.
Other prior art attempts to provide a controlled dosage release is described in U.S. Pat. No. 3,247,066. In this patent a device is described comprised of a polymeric wall that fully surrounds a drug colloid mixture. Drug is released from the device under the influence of water entering the mixture to cause an increased internal swelling pressure of the colloid that builds up sufficiently to exceed the strength of the polymer wall. The build-up is followed by a rupture of the wall that immediately releases drug in an undetermined amount. This action, plus any subsequent drug release, is by seemingly uncontrolled dissolution which features tend to inherently limit the acceptability of this device for the controlled and prolonged administration of drug. Similarly, in U.S. Pat. No. 3,538,214 there is disclosed a tablet comprised of a drug matrix coated with a microporous film permeable to fluid and drug as another prior art means for the controlled release of drug. The device of this patent releases drug by letting water pass through the pores to dissolve drug which is released through the pores. The rate of drug release is again influenced by environmental factors such as pH dependency and this coupled with both an unspecified number of pores of variable sizes arising through manufacture and material limitations limits the use of this device in the useful arts and sciences.
In copending patent application U.S. Ser. No. 259,469, filed on June 5, 1973, now U.S. Pat. No. 3,845,770 and U.S. Patent application Ser. No. 440,281, filed on Feb. 7, 1974, now U.S. Pat. No. 3,916,889, which applications are assigned to the same assignee of this patentable invention, there are disclosed novel and useful dispenser devices that represent both a pioneer contribution and improvement to the dispensing art. While the devices of that invention give outstanding, operable and useful results and release their agent at an osmotically controlled rate, it has now been found that a unique means for letting the device release their agent only in certain environments or for changing the release rate pattern of the agent can be manufactured into the device. It will be appreciated by those versed in the art to which the invention pertains that this means not only increases the usefulness of the device but enhances its range of operation as a useful article of manufacture.